Scientists said Wednesday that flu infections were rising among pigs raised for slaughter on farms in south and southeastern China, also plagued by bird flu.
And the risk of spillover to humans was "constant or growing", according to one of the authors of a study published in Proceedings of the Royal Society B.
Pigs are an important source of new human strains of influenza A, such as the 2009-10 H1N1 pandemic that emerged in Mexico and infected an estimated fifth of the world's population.
Pigs can act as a "mixing vessel" in a process known as reassortment, brewing new flu strains from swine, poultry and human viruses in areas where they live in close proximity.
Such new hybrids can be deadly -- tens of millions of people died in flu pandemics in 1918, 1957 and 1968.